On a vehicle having an engine mounted in a frame with at least one front wheel and at least one driven rear wheel pivotally mounted to the aft end of a swingarm, said swingarm having its forward end pivotally attached to the frame, and the power being transmitted to the rear wheel through a chain and sprocket arrangement, the current art is to mount the forward end of the swingarm to the frame at a location aft of the engine driver sprocket. The chain and sprocket arrangement consists of an engine driver, or countershaft sprocket, a rear driven sprocket attached to the rear wheel or rear axle, and an endless drive chain engaged around both the countershaft and driven sprockets. One end of a spring and shock absorbing device is also mounted to the swingarm and the other end of the device is attached to the frame. This arrangement allows the driven wheel and swingarm to move vertically, above and below, a normal straight-line disposition of engine countershaft sprocket, swingarm forward pivot, and rear wheel axle as the vehicle transverses rough or uneven terrain.
The distance from the centerline of the countershaft sprocket to the centerline of the driven sprocket, as measured at the normal straight-line disposition, is hereafter referred to as to the theorical radius.
Whereas, the forward pivotal end of the swingarm attaches to the frame at a location aft of the countershaft sprocket. This creates a condition where the actual arc generated by the sweep of the rear wheel as it moves vertically, upward or downward, from the straight-line position does not coincide at every point with the theorical arc generated by the theorical raduis as the rear wheel moves through its total amount of travel above and below the straight-line position. The two arcs, the actual and the theorical, only coincide at one point. This is when the swingarm is in its straight-line disposition. As the rear wheel moves vertically upward or downward from the straight-line position, the chain becomes slack because the actual radius is less than the theorical radius. It follows that the greater the amount of actual movement of the rear wheel above or below the straight-line position, the greater the amount of chain slack.
The invention proposes a mechanism that will allow the forward pivotal end of the swingarm to be mounted at a location aft of the engine countershaft sprocket, as is the current art, yet the invention also allows the rear wheel and drive sprocket to follow the theorical arc as generated by the theorical radius, thus keeping an ever taut chain.
The mechanisms required to maintain an ever taut chain consist of (1) an eccentric cam and attached lever arm, (2) a rocker lever, (3) a connecting rod from the cam lever arm to the rocker lever and (4) a connecting rod from the rocker lever to the swingarm. The cam is pivotally mounted to the frame at a location aft of the countershaft sprocket with the offset of the cam pointed directly toward the countershaft sprocket when the swingarm is in its straight-line position. The cam lever arm is torsionally attached to the cam and extents generally upward and rearward from the cam. The forward end of the swingarm is pivotally mounted to the cam and constructed so as not to interfere with the cam lever arm as the swingarm and cam move through their respective motions. The mounting provision for the swingarm connecting rod is located on the swingarm at a point between the forward pivotal mount and the rear axle mount.
The rocker arm consists of a member with provisions for a pivotal mount located on either end of the arm and a pivotal mount located between the two ends. The rocker arm is pivotally attached to the frame at a location on the frame generally above and aft of the cam pivot location. Attachment of the rocker arm to the frame is made using the middle pivotal mount of the rocker arm. The swingarm connecting rod is attached to the aft end pivotal mount of the rocker arm. The cam connecting rod is connected between the forward end of the rocker arm and the cam lever arm. The cam connecting rod is pivotally attached to both the rocker arm and the cam lever arm.
The intent of the above described members is to allow the swingarm to move at both the forward end and the aft end, but not equally, in such a manner that the aft and of the swingarm at the rear wheel pivot location follows the path of the theroical arc. The forward end of the swingarm moves in the same direction, up or down, as does the rear of the swingarm during the movement of the rear wheel. The movement of the forward end of the swingarm, however, is controlled by the movement of the cam. As the cam rotates on the pivot bolt, it forces the forward end of the swingarm to move, either up or down, depending on the aft end movement. As the cam forces the forward end up or down, the cam also moves the swingarm backwards a small amount. It is both the upward and backward movement that allows the rear of the swingarm to follow the arc created by the theroical radius.
The movement of the cam is controlled by the movement of the swingarm through the interconnected components. As the aft end of the swingarm moves upward from the straight line dispostion, the swingarm connecting rod forces the aft end of the rocker arm to also move upward. The forward end of the rocker arm, however, moves downward and through the cam connecting rod forces the cam lever arm down which in turn forces the cam to pivot upward. Exactly the opposite movements would result if the aft end of the swingarm moves downward.
The relative angular movement of the cam with respect to the angular movement of the swingarm is critical. Too much movement of the cam would force the rear of the swingarm beyond the length of the theorical raduis, and too little movement of the cam would not keep the chain taut. To achieve the correct amount of angular movement of the cam, all of the individual components are sized, based on their trigonometric relationship with the rest of the components. Once the components are correctly sized the invention maintains a taut chain regardless of the position of the rear end of the swingarm.